1. Field of the Invention
The invention relates to a method and a device for simultaneous compensation of signal errors in IQ modulators.
2. Related Technology
IQ modulators are used in modern data transmission systems for the modulation of unmodulated high-frequency carriers. By contrast with traditional modulation methods, which generate a real-value bandpass signal, IQ modulators can, in principle, generate a complex bandpass signal in the intermediate-frequency or high-frequency range. Accordingly, the generation of any required vectors in the intermediate-frequency or high-frequency range allows a better exploitation of the available bandwidth by comparison with a real-value modulation method.
In practice, especially in the case of IQ modulators realized in an analog manner, generating any vector at the complex intermediate-frequency or high-frequency level with an accuracy required for data transmission is problematic. Because of the analog circuit realisation of the IQ modulators, undesirable static signal errors occur, which are superimposed in a static manner over the individual input signals of the IQ modulators and lead to a permanently error-laden complex signal at the output of the IQ modulator, which can be detected in the complex status diagram as a displacement of the vector of the complex output signal.
In essence, the following static signal errors can be listed:                input offset error: with an uncontrolled, in-phase and quadrature-phase channel, the baseband signals of the in-phase and quadrature-phase channel provide a value other than zero and cause a non-optimal attenuation of the respective carrier signal;        high-frequency crosstalk: the carrier signal talks over the bandpass signal at the output of the IQ modulator via the two multiplication units of the IQ modulator;        non-linear modulation: different amplitudes occur in the side bands of the complex baseband signal, because of unavoidable non-linearities—especially non-linearity-differences—between the two multiplication units of the IQ modulator;        residual carrier: one side band of the bandpass signal is incompletely attenuated for a single-sideband transmission because of an incorrectly-dimensioned bandpass filter at the output of the IQ modulator;        amplification error in the baseband: the sensitivity of the amplification elements at the in-phase and quadrature-phase input of the IQ modulator is incorrectly adjusted and/or calibrated and, in particular, is designed asymmetrically;        amplification error in the multiplication units: the sensitivity of the two multiplication units of the IQ modulator is incorrectly adjusted and/or calibrated and, in particular, is designed asymmetrically;        quadrature error: the output signals of the two multiplication units of the IQ modulator are not mutually orthogonal because of phase distortions of the two carrier signals of the in-phase and quadrature-phase signal;        phase error: the in-phase and quadrature-phase signal do not have the same phase error, for example, because of an incorrect carrier recovery or clock-pulse synchronisation.        
Since several of these named, static signal errors have an identical effect on the bandpass signal at the output of the IQ modulator and are superimposed in a linear manner and cannot therefore be identified separately by the measurement technology, it is meaningful to combine static signal errors with the same effect on the bandpass signal into signal error types. In principle, there are three types of static signal error:                signal errors with additive effect on the bandpass signal: input-offset errors, high-frequency crosstalk, non-linear modulation, residual carrier;        signal errors with multiplicative effect on the bandpass signal: amplification errors in the baseband, amplification errors in the multiplication units;        signal errors with effect on the phase of the bandpass signal: quadrature errors, phase errors.        
All of the listed static signal errors lead to an incorrect interpretation of the transmission signal in the receiver of the data transmission system. If it is not possible to minimise the effect of these signal errors, which reduce the quality of the data transmission, restricting the data-transmission bandwidth is the only expedient solution. Since these static signal errors can hardly be removed at an economically-viable cost by means of circuit technology, and restricting the data-transmission bandwidth is generally not acceptable, the only feasible goal is to compensate such signal errors by means of compensation or correction networks.
For example, DE 199 34 215 C1 presents an arrangement for the compensation of static signal errors, which are generated in IQ modulators. Corresponding to the three types of signal errors, an adding unit and a multiplication unit with additive and multiplicative effect is integrated in each case in the in-phase and quadrature-phase channel in order to compensate the signal errors. To remove the signal errors with effect on the phase of the bandpass signal, an additional control input is provided on the phase modifier in order to realise mutually-orthogonal carrier signals. The two addition and multiplication units and also the control input on the phase modifier are controlled via a controller with corresponding correction values in order to compensate the individual signal errors. A reconstruction of the in-phase and quadrature-phase signal from the bandpass signal at the output of the IQ modulator via an equivalent IQ demodulator and an appropriate implementation of control algorithms within the controller allows the compensation of static signal errors.
An optimum compensation of the individual signal errors requires the absolute measurement of each individual signal error or alternatively the measurement and/or determination of each individual effective signal error as a difference between the individual signal errors and the associated correction signal. This is not possible with the arrangement disclosed in DE 199 34 215 C1, because the error-laden in-phase and quadrature-phase signals recovered in the IQ demodulator are supplied to the controller as actual-value signals. A measurement of the individual signal errors from the in-phase and quadrature-phase signals recovered is not disclosed in that document. Additionally, for an optimum compensation of individual signal errors, the associated correction errors must be generated in a decoupled manner for each individual signal error on the basis of the difference between the signal error and the correction signal. This is also not disclosed in the document DE 199 34 215 C1, because this document does not disclose a compensation of the respective effective signal error in a decoupled manner for each individual signal error as a mutually-decoupled compensation of the difference between the individual signal error and the respectively associated correction signal in the description of the controller.